Such a method is known from ZELLERHOFF, M. et. al., “Low contrast 3D-reconstruction from C-Arm data”, Proceedings of SPIE, Medical Imaging 2005, Vol. 5745, pages 646 to 655. The disclosed method is used to prevent truncation artifacts which occur if the object under examination extends into areas outside the so-called scan field of view. The resulting projection images are termed cut-off or truncated. Truncated projection images produce artifacts when the slices are reconstructed. In particular the image values near the edge in the slices are generally too high and too low in a central region. The slices affected by truncation artifacts are therefore of only limited diagnostic use.
In the known method, there is constructed in the edge region of a projection image, if attenuation is present there, an equivalent body which produces the same attenuation in the edge region as the object under examination. Assuming parallel beam geometry, the equivalent body is then projected on the region outside the projection image, causing the projection image to be continued in a region outside the projection image.
The equivalent body is projected onto regions outside the projection image using parallel beam geometry, even though a divergent beam geometry, e.g. a fan beam, is actually present. The equivalent body is also adapted to the image values in the edge region of the projection image on the basis of a parallel beam geometry. In this respect, errors in correcting the truncation artifacts may be induced.
The known method has the advantage, however, that no resorting of the fan beam data into parallel beam data is necessary. Resorting of the fan beam data into parallel beam data is also termed rebinning. So-called rebinning is very compute intensive and cannot be used in every case. Particularly in the case of C-arm computed tomography recordings, the wait and compute times required for this purpose are not available because of the already very long image reconstruction times.
A method in which resorting of the fan beam data into parallel beam data is performed is known from HSIEH, J. et al., “A novel reconstruction algorithm to extend the CT scan field-of-view”, MED. PHYS. 31 (9), September 2004, pages 2385 to 2391. After resorting, an equivalent body is reconstructed and the equivalent body is projected with parallel beam geometry onto regions outside the projection image. In this respect this is a correct method from a theoretical standpoint.